In addition to high test and regular gasoline and diesel fuel, alcohols, such as methanol, ethanol, propanol, isobutanol, etc. are suitable as fuels for internal combustion engines.
The calorific value of the combustible fuel-air mixture is a determining factor for the power of the engine. For a stoichiometric mixing ratio, the calorific value of the mixture for substantially all utilizable liquid fuels and liquified petroleum gases lies between 700 and 735 Kcal/kg; they are thus almost equal and engine power losses should not be expected by mixing methanol or other alcohols into the fuel mixture. In addition, most of the other properties of alcohols are such that they can be mixed with common liquid fuels without anything further and they cause no disturbances in practical engine operation.
An alcohol composition is particularly desirable for the reason that alcohols are suitable as a refining fuel for low-grade gasolines. By adding, for example, methanol to regular gasoline, the anti-knock property of the gasoline can be substantially improved, so that with this mixture even high compression internal combustion engines can be operated. Therefore, it becomes unnecessary to add anti-knocking agents to the fuel, which may be harmful to the environment.
Therefore, alcohols are becoming of increasing importance in the USA for environmental reasons, particularly due to the stricter regulations in California.
In addition, alcohols, particularly methanol, can be produced from coal relatively cheaply, and alcohols will thus be available in large quantities well into the distant future. In particular, ethanol is environmental-friendly, if it is obtained from plant products. Alcohols are thus a suitable supplement for fossil fuels whose supply is ultimately limited.
However, it is disadvantageous that the specific minimum air requirement for complete combustion in the case of alcohols is less than for conventional fuels. For the same aspirated air quantity, a correspondingly higher quantity of fuel must be introduced to the engine in the case of alcohol or for a mixture of gasoline and alcohol in order to obtain a stoichiometric air-fuel ratio. This makes necessary a corresponding adjustment of the mixture-forming means, i.e. fuel injection or carburation systems of the internal combustion engine. Moreover, since the alcohols are usually added in varying amounts, as necessary, or can vary in the vehicle fuel tank when different types of fuel are added to the tank, the required air-fuel ratio for a stoichiometric mixture must be determined anew in the electronic control device for the mixture-forming means according to the alcohol fraction in the fuel tank.
In DE-OS 40 19 188 there is disclosed a multicomponent engine control with a fuel sensor for determining the mixture fraction of methanol in a fuel-methanol mixture, whose output signal is used for subsequently establishing a stored engine fuel-control dielectric sensor, which contains two electrode plates immersed in the fuel and arranged in spaced relation from one another. The dielectric constant of the fuel is detected by measurement of the capacitance between the two electrode plates, the added fraction of methanol being derived from the dielectric constant.
This measurement system, however, has a high sensitivity to vapor lock. In addition, this fuel sensor is only suitable to detect the mixture fraction of a previously known alcohol (e.g., methanol).
A device for automatic adaptation of an internal combustion engine to the requirements of the selective use of various types of liquid fuels is known from DE-OS 22 59 323 which has a closed measurement tank for flow therethrough of fuel from an inlet to an outlet. In the measurement tank a partial quantity of fuel is vaporized under the effect of applied heat, whereby the relative size of the vapor-filled space inside the measurement tank serves as a measurement for determining the different types of fuels.
A disadvantage of this system is that it can only be established whether a specific fraction has been exceeded or fallen below, whereby even here this fraction must be known beforehand in order to arrange the measurement sensor of this device at a specific level in the measurement tank.